The purpose of this research project is to define the biological and clinical significance of LDH-X, a unique form of lactate dehydrogenase in the sperm of a variety of mammalian and avian species. Experiments have been designed to answer the following questions: 1. Do changes in the qualitative or quantitative characteristics of LDH-X correlate with histologic appearance of the testis, as well as the number and type of sperm in seminal fluid? 2. Can LDH-X activity be used as a measure of the testicular effects of male antifertility compounds, including cancer chemotherapeutic agents such as cyclophosphamide, methotraxate, and vincristine? What are the short term and long term effects of these compounds on testis and spermatogenesis? Are qualitative and quantitative measurements of LDH-X activity a more sensitive indicator of long term effects of these agents? 3. What factors control the activity of the LDH C locus, the structural locus responsible for the synthesis of LDH-X? 4. Is vasectomy in the male associated with the development of LDH-X antibodies in the serum? Hopefully, the results of these studies will answer questions regarding the biological significance of LDH-X and in particular, how the enzyme relates to certain types of male infertility. Also, the results of these experiments may provide insight into factors regulating the activity of certain genetic loci. Here again, the definition of these factors may advance our knowledge of the molecular biology of reproduction.